Paper Formating Guidelines

Deeply understanding HBT’s thermal characteristics and safe operating area (SOA) is critical to the improvement of power amplifier’s robustness and reliability. In this paper, first, we measured DC IV curves for our HBT unit cells with various sizes at the current density up to over 300kA/cm2. Due to the observed unconventional phenomenon at high injection level, we defined SOA in terms of the maximum power density, which is directly associated with the thermal effect of the unit cell. We found that the SOA improves with reducing unit cell size in nearly all range of power densities. To further study this thermal size effect, we performed 3D thermal simulation for the unit cells with various sizes and layouts including all device details such as epi and metal layers. We found that both the thermal resistance and junction temperature non-uniformity generally increase with emitter size. The impact of the temperature non-uniformity within the finger on the thermal “runaway” is discussed for the first time. Also our analytical model with the electrothermal and impact ionization mechanisms agrees well with the experimental finding and 3D simulation results. Fig. 1. A typical measured DC collector current density vs collector voltage (2E1B Ae=2×45um) (a) base voltage controlled: Vb from 1.28 to 1.6V and (b) base current controlled: Ib from 2uA to 6mA. Each curve goes all the way until the device dies. We can see, at high Ib or Vb, the curves cross over each other implying the thermal instability and particularly for Vb-controlled, the current peaks and then drops before the “fly-back”. This unconventional phenomenon can be clearly shown with our analytical model. 0 50 100 150 200 250 0 2 4 6 8 10 12 14 16 Jc (k A/ cm 2 ) Vce (V) Ib=2uA Ib=20uA Ib=100uA Ib=200uA Ib=400uA Ib=600uA Ib=800uA Ib=1mA Ib=1.3mA Ib=1.6mA Ib=2mA Ib=3mA Ib=4mA Ib=6mA 0 20 40 60 80 100 120 140 160 180 200 0 2 4 6 8 10 12 14 Jc (k A/ cm 2 ) Vce (V) Vb=1.6V Vb = 1.55V Vb=1.5 Vb = 1.45V Vb=1.40V Vb=1.35V Vb=1.34